Electrochemical dimerization of beta-halopropionitriles in aqueous media



United States Patent 3,475,298 ELECTROCHEMICAL DIMERIZATION 0F p-HALO- PROPIUNITRILES IN AQUEOUS MEDIA Sam Andreadcs, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware No Drawing. Filed May 31, 1966, Ser. No. 553,669 Int. Cl. B01k 3/.00

U.S. Cl. 204-73 4 Claims ABSTRACT OF THE DISCLOSURE Certain B-halopropionitriles can be electrolyzed in aqueous media to yield corresponding adiponitriles. fl-chloropropionitrile for example can be electrolyzed to adiponitrile.

This invention relates to, and has as its principal object provision of, a process for the production of adiponitrile or substituted adiponitriles by the electrochemical dimerization of fi-halopropionitriles.

In accordance with the present invention, it has been found that certain B-halopropionitriles can 'be electrolyzed in aqueous media to yield, at the cathode, corresponding adiponitriles. An equation for the cathode reaction can be written as:

Water of the aqueous medium takes part in the overall reaction, liberating oxygen at the anode, and an equation therefor can be written as:

2XCH CHRCN+H O+ (CH CHRCN) 2 +2HX+ /2 T0 In these equations, X represents halogen of atomic number 17-53, i.e., chlorine, bromine or iodine, and R is hydrogen, alkyl of 1-8 carbons, phenyl or 0-, mor p-substituted phenyl where the substituents are CN, Cl, Br, alkyl or 1-8 carbons or alkoxy of 1-8 carbons. Mixed products, i.e., products in which the two Rs are different, can, of course, be prepare by using more than one p-halopropionitrile in the same solution.

In carrying out the above process, the initial concentration of total ,B-halopropionitrile in the aqueous catholyte should be between 540% of the solution, -40% or a solution saturated with 8-halopropionitrile being preferred. All percent-ages herein are by weight based on electrolyte constituents excluding ,B-halopropionitrile.

The apparatus in which the electrolysis is carried out can be a conventional electrolysis cell. Thus a glass container may be divided into catholyte and anolyte compartments by means of a porous diaphragm made, for example, of Alundum. A mercury pool with appropriate electrical connections provides a convenient cathode but other electrodes having high hydrogen overpotentials may also be used, e.g., those made from zinc, lead, tin, cadmium, copper, and alloys thereof, etc. If a mercury pool is employed as the cathode, a magnetic stirring bar can be floated on the pool to insure mixing and minimize polarization effects. If the anolyte is separated from the catholyte to avoid oxidation of products or reactants at the anode or by anode products, any conductive anode material may be used, e.g., carbon, platinum, copper, etc.

Both the anolyte and the catholyte preferably are aqueous solutions. The anolyte may be composed of any electrolyte which conducts current readily. For convenience, the same aqueous salt solutions used as the catholyte may be used as the anolyte, the fl-halonitrile react-ant being omitted in the anolyte. The concentration of salt used as the joint electrolyte should be between about 10 and 50% by weight, -50% being preferred. Higher concentrations of electrolytes increase conductivity and minimize necessary applied potential.

Among the salts which can be employed as the joint electrolyte according to the invention, the amine and quarternary ammonium salts are generally suitable, especially those of sulfonic and alkyl sulfuric acids as long as the pH of the catholyte is within the limits outlined below. The saturated, aliphatic or heterocyclic quaternary ammonium salts are well adapted to the process since they have sufiiciently high cathode discharge potentials and readily form water-soluble salts with anions suitable for use in the electrolytes employed. It is, of course, undesirable that the ammonium groups contain any reactive groups which might interfere to some extent with the electrolysis reaction. Suitable salts are the saturated aliphatic amine salts or heterocyclic amine salts, e.g., the mono-, di-, or trialkylamine salts, or th mono-, dior triakanolamine salts, or the piperidine, pyrrolidine or morpholine salts, e.g., the ethylamine, dimethylamine or triisopropylamine salts of various acids, especially various sulfonic acids. Especally preferred are aliphatic and heterocyclic quaternary ammonium salts, i.e., the tetraalkylammonium or the tetraalkanolammonium salts or mixed alkyl alkanol ammonium salts such as the alkyltrialkanolarnmonium, the dialkyldialkanolammonium, the akanoltrialkylammonium or the N-heterocyclic N-alkyl ammonium salts of sulfonic or other suitable acids.

Among the anions useful in the salts of the electrolytes, those of the aryl and 'alkaryl sulfonic acids are especially suitable, for example, salts of the following acids: benzenesulfonic acid, o-, mor p-toluenesulfonic acid, o-, m-, or p-et-hylbenzenesulfonic acid, o-, mor p-cumenesulfonic acid, o-, m-, p-tertamylbenzensulfonic acid, 0, m-, or phexylbenzenesulfonic acid, oxylene-4-sulfonic acid, p-xylene-Z-sulfonic acid, m-xylene-4 or 5-sulfonic acid, mesitylene-Z-s-ulfonic acid, durene-Z-sulfonic acid, pentamethylbenzenesulfonic acid, o-dipropyl-benzene-4-sulfonic acid, aor ,B-naphthalenesulfonic acid, o-, m-, or p-biphenylsulfonic acid, and a-methyl-fl-naphthalenesulfonic acid. Alkali metal salts, i.e., the sodium, potassium, lithium, cesium or rubidium salts, of such sulfonic acids can be employed with certain limitations. Such salts include sodium benzene-sulfonate, potassium p-toluenesulfonate, lithium o-biphenylsulfonate, rubidium fi-naphthalene-sulfonate, cesium p-ethylbenzenesulfonate, sodium o-xylene- 3-sulfonate, and potassium pentamethylbenzenesulfonate. Saturated, aliphatic amine or heterocyclic amine salts of such sulfonic acids may also be used. Examples are the mono-, dior trialkylamine salts, or the mono-, dior trialkanolamine salts, or the piperidine, pyrrolidine, or morpholine salts, e.g., the ethylamine, dimethylamine or triisopropylamine salts of benzenesulfonic acid or of o-, por m-toluenesulfonic acid; the isopropanolamine, dibutanolamine or triethanolamine salt of o-, por m-toluenesulfonic acid or of o-, por m-biphenylsulfonic acid, the piperidine salt of aor B-naphthalenesulfonic acid or of the cumene-sulfonic acids; the pyrrolidine salt of o-, m-, or p-amylbenzenesulfonate; the morpholine salt of henzensulfonic acid, of o-, m-, or p-toluenesulfonic acid, or of aor fi-naphthalenesulfonic acid, etc.

In general, the sulfonates of any of the ammonium cat- 10I1S disclosed generically or specifically herein can be employed in the present invention. Useful quaternary ammonium sulfonates are, e.g. tetraethylammonium o-, or m-toluenesulfonate or benzenesulfonate; tetraethylammonium o-, mor p-cumenesulfonate, or o-, m-, or p-ethylbenzenesulfonate, tetramethylamrnonium benzenesulfonate, or o-, mor p-toluenesulfonate; N,N-di-methylpiperi dinium o-, mor p-toluenesulfonate or o-, mor p-biphenylsulfonate; tetrabutylammonium aor fl-naphthalenesulfonate or o-, mor p-toluenesulfonate; tetrapropylammonium mor p-amylbenzenesulfonate or a-ethylfl-naphthalenesulfonate; tetraethanolammonium o-, mor p-cumenesulfonate or o-, m or p-toluene sulfonate; tetrabutanolammonium benzenesulfonate or p-xylene-3-sulfonate; tetrapentylammonium o-, mor p-tolnenesulfonate or o-, mor p-hexylbenzenesulfonate, tetrapentanolammonium peymene-3-sulfonate or benzenesulfonate; methyltriethylammonium o-, m or p-toluenesulfonate or mesitylene-2-sulfonate; trimethylethylammonium o-xylene-4- sulfonate or o-, m-, or p-toluenesulfonate; triethylpentylammonium aor B-naphthalenesulfonate or o-, mor pbutylbenzenesulfonate, triethylethanolammonium benzenesulfonate or o-, mor p-toluenesulfonate; N,N-diethylpiperidinium or N-methylpyrrolidinium, o-, mor p-hexyL benzenesulfonaate or o-, mor p-toluenesulfonate, N,N- di-isopropyl or N,N-dibutylmorpholinium o-, 1nor p-toluenesulfonate or o-, m or p-biphenylsulfonate, etc.

Electrolyses using tetraalkylammonium salts of the aryl or alkarylsulfonic acids are exclusively electrochemical processes. On the other hand, with the alkali metal sulfonates yields are markedly lower than those obtained with tetraalkylarnmonium sulfonates even when there is present in the catholyte the high concentration of ,B-halopropionitrile attainable by employing a co-solvent such as ethanol or tetrahydrofuran. Lower yields probably occur because at cathode voltages necessary for the electrolysis, the alkali metal salts are also affected and the alkali metal ions are discharged to some extent. When tetraalkylammonium sulfonates are used, no appreciable amount of tetraalkylammonium ion is dischaged.

Among the ammonium and amine sulfonates useful as electrolytes in the present invention are the alkyl, aralkyl, and heterocyclic amine and ammonium sulfonates, in which ordinarily the individual substituents on the nitrogen atom contain no more than atoms, and usually the amine or ammonium radical contains from 3 to carbon atoms. Diand poly-amines and diand poly-ammonium radicals are also operable and are included by the terms amine and ammonium. The sulfonate radical can be from aryl, alkyl, alkaryl or aralkyl sulfonic acids of various molecular weights up to for example 20 carbon atoms, preferably about 6 to 20 carbon atoms, and can include one, two or more sulfonate groups.

Another class of salts suitable for use in the present invention are the alkylsulfate salts such as methosulfate salts, particularly the amine and quaternary ammonium methosulfate salts. The amine and ammonium cations suitable for use in the alkylsulfate salts are the same as those for the sulfonates. Any of the quaternary ammonium alkylsulfates, such as tetraethylammonium ethylsulfate, can be utilized. Methosulfate salts such as the methyltriethylammonium, tri-n-propropylmethylammonium, triamylmethylammonium, and tri-n-butylmethylammonium salts are very hygroscopic, and the tri-n-butylmethylammonium salt in particular forms very concentrated aqueous solutions which dissolve large amounts of organic materials.

Various other cations are suitable for use in the present invention, e.g., tetraalkylphosphonium and trialkylsulfonium cations, particularly as sulfonate salts formed from sulfonic acids as described above, or as methosulfate salts.

As a further illustration of electrolytes suitable for use in the present invention, the following named salts are all suitable for electrolyzing fl-chloropropionitrile to obtain adiponitrile as the major product generally employing concentrated aqueous solutions of the salts containing at least 5% and usually 20 to 40% by weight of B-chloropropionitrile, and utilizing the general procedures of the illustrative examples herein:

(1) N trimethyl-N'-trimethylethylenediammonium di-ptoluenesulfonate;

(2) Benzyltrimethylammonium p-toluenesulfonate;

(3) Methyl-tri-n-butylphosphonium p-toluenesulfonate;

(4) Tetraethylammonium sulfate;

(5) Di-tetraethylammonium benzenephosphonate;

(6) Trimethylsulfonium p-toluenesulfonate;

(7) Methyl-tri-n-hexylammonium p-toluenesulfonate;

(8) Benzyltrimethylammonium phosphate;

(9) Benzyltrimethylammonium acetate;

(10) Methyl-tri-n-butylammonium methosulfate;

(l1) Benzyltrimethylammonium benzoate;

(12) Tetraethylammonium methanesulfonate;

(13) Benzyltrimethylammonium Z-naphthalenesulfonate;

(14) Bis benzyltrimethylammonium m-benzenedisulfonate;

(15) Benzyltrimethylammonium thiocyanate; and

(16) Tetramethylammonium methosulfate.

Various other quaternary ammonium, tetraalkylphosphonium or trialkylsulfonium salts can be employed, in the process, e.g., the halides, sulfates, phosphates, phosphonates, acetates, benzoates, and other carboxylic acid salts, specifically, for example, tetramethylammonium bromide, tetraethylammonium bromide, tetramethylammonium chloride, tetraalkylphosphonium chloride, tetraethylammonium phosphate, etc. Similarly the alkali, alkaline earth and other metal salts with the foregoing anions can be employed, e.g., sodium chloride, potassium phosphates, sodium acetate, calcium acetate, lithium benzoate, cal cium chloride, rubidium bromide, magnesium chloride, as well as the sulfonic acid, particularly aromatic sulfonic acid, and alkylsulfuric acid salts of the foregoing cations and of other alkali, alkaline earth, rare earth and other metals, e.g., cesium, cerium, lanthanum, yttrium, particularly with anions to achieve sufficient water solubility. The solutions designated herein as containing salts, electrolytes, etc., in specified amounts have reference to solutions containing salts sufficiently stable to remain substantially in solution. It will be recognized that many cations are capable of existing in several valence states, and some valence states will be more suitable as supporting electrolytes than others. Other examples of salts which can be employed in the present process, although not necessarily with equivalent or optimum results, are barium bromide, barium acetate, barium propionate, barium adipate, cerium sulfate, cesium chloride, cesium benzoate, cesium benzenesulfonate, potassium oxalate, potassium sulfate, potassium ethyl sulfate, lanthanum acetate, lanthanum benzene sulfonate, sodium sulfate, sodium potassium sulfate, strontium acetate, rubidium sulfate, rubidium benzoate, trisodium phosphate, sodium hydrogen phosphate and sodium bicarbonate.

Voltage at the cathode during the electrolysis should be maintained at about 1.5 to 2.3 volts as referred to a saturated calomel electrode. Any steady source of direct current can, of course, be employed to maintain this voltage. Temperature and pressure are not critical and are usually ambient for the sake of convenience.

Perhaps one of the most critical parameters to be observed during the operation of the present electrolytic process is the pH of the catholyte. This should be maintained in the range 5-7, around 7 being preferred. Gas evolution (hydrogen) and the formation of propionitriles occur at the cathode with resultant loss in efiiciency as the pH decreases. A pH higher than 7 tends to cause a chemical elimination of hydrogen chloride from the ,6- chloropropionitriles, although adiponitriles are still formed. Since the pH drops in the course of the electrolysis, additions of a basic reagent are necessary to maintain neutrality. These materials may include hydroxides, acetates, formates, and the like. However, the associated cations should preferably be cations which are not reducible at the mercury electrode at the operating potentials, e.g., tetraethylammonium, tetra-n-butylammonium, tetraphenylarsonium, and triethylammonium ions.

In one embodiment of the invention an anion permiselective membrane is used to separate the anode and cathode compartments. In this embodiment, chloride ion produced in the catholyte compartment migrates to the anolyte compartment while the migration or protons from anolyte to catholyte is restricted. The chloride ion in the anolyte is then oxidized to chlorine gas which may be collected at the anode.

There follow some nonlimiting examples which illustrate the invention in greater detail. In these examples, unless otherwise noted, pressures and temperatures are ambient and parts and percentages are by weight. Temperature throughout the specification is expressed in degress centigrade. Electrode potentials are given relative to a saturated calomel electrode (S.C.E.).

EXAMPLE 1 (A) Electrolysis apparatus was set up consisting of a glass vessel divided into unequal compartments for the anolyte and the catholyte by an Alundum membrane. The cathode was a mercury pool in which floated a magnetic stirring bar serving to minimize polarization effects. Heat generated by cell resistance was dissipated by a water-jacket on the cathode compartment and by a watercooled coil inserted into the catholyte. A thermometer and a saturated calomel reference electrode were inserted into the catholyte so the reference electrode was in close proximity to the mercury pool. In this manner, at a current level of about 2 amperes, the operating temperature of the catholyte was 20-25.

(B) The anolyte compartment was filled with 40 ml. of 50% aqueous tetramethylammonium p-toluenesulfonate. The catholyte compartment was filled with a solution from 60.0 g. (0.33 mole) of tetramethylammonium hydroxide pentahydrate, 50 g. of distilled water, 63.0 g. (0.33 mole) of p-toluenesulfonic acid monohydrate, and 45.0 g. (0.50 mole) of B-chloropropionitrile. The electrolyte was carefully adjusted to pH 6.5-7.0 before the nitrile was added by adding either the above base or above acid, whichever was required.

The catholyte solution was then electrolytically reduced at a cathode potential of 2.1 to 1.8 v. (S.C.E.) over a period of 6.5 hours. The current level changed during the latter part of the run from 3 amperes to about 5 amperes. The total voltage drop across the cell varied from 10.0-8.5 v. and as the pH dropped at times to about 3, 'tetramethylammonium hydroxide was added to restore the solution to neutrality. The total current consumption was 0.7 Faraday. Gas evolution occurred at the cathode when the pH was low. At pHs near 7, no cathodic gas evolution was noted. The gas was collected and identified as hydrogen.

At the end of the electrolysis, the catholyte was placed in a liquid-liquid extractor with methylene chloride as the extracting solvent and kept there overnight. The extract was dried and distilled to give 31.5 g. of liquid products. The distillation products were analyzed for p-chloropropionitrile and adiponitrile by gas phase chromatography and were resolved into the components by a second distillation or by preparative gas chromatography. In this manner, it was found that the yield of adiponitrile was 57% and the yield of propionitrile was 43%. Acrylonitrile (0.1%) was also found. A current efl'iciency of 41% for conversion to adiponitrile was calculated.

In experiments similar to the above tetramethylammonium acetate was added as a bufier to maintain the pH at about 5.

EXAMPLE 2 A reduction of 3-chloropropionitrile was carried out using the apparatus and procedure described above except that 50.0 g. (0.056 moles) of fi-chloropropionitrile was used and the cathode potential was maintained at 2.75 v. After 7 hours, the potential had dropped to 2.1 v. and was maintained at this level for an additional 5.0 hours. A total of 1.0 Faraday was consumed. This experiment confirmed the fact that cathodic gas evolution increased as the pH dropped and decreased as the catholyte was neutralized by the addition of tetramethylammonium hydroxide.

At the end of the electrolysis, the catholyte was extracted six times with 60-ml. portions of methylene chloride. The combined extracts were dred and distilled using a Dry-Ice trap to give approximately 4.12 g. of product which was largely propionitrile along with some acrylonitrile. Continued distillation gave Fraction 1, B.P. 49-5 8 (57 mm., 1.6 g.) which was mainly acrylonitrile and propionitrile; Fraction 2, =B.P. 48 (57 mm.) to 44 (2.1 mm., 6.2 g.) which was mainly propionitrile; Fraction 3, B.P. 4470 (2.1 mm., 0.2 g.) which was mainly ,B-chloropropionitrile and Fraction 4, B.P. 116-122 (20 mm., 7.0 g.) which was adiponitrile. The yield of adiponitrile was 23%, and the yield of propionitrile was 33%.

EXAMPLE 3 A neutral solution containing 63.0 g. of tetramethylammonium hydroxide pentahydrate, 50 g. of distilled water, and 63 g. of p-toluenesulfonic acid monohydrate was combined with 40 g. of commercial B-chloropropionitrile. The nitrile was strongly acidic (pH ca. 2), and reduced the pH of the solution to ca. 5. Large additions of tetramethylammonium hydroxide pentahydrate were required to elfect small changes in pH and addition of 70.0 g. was required to bring the pH back to ca. 7. The anolyte was composed of 14 g. of the hydroxide, 12 g. of the acid, and 10 g. of distilled water. Electrolysis was carried out at a mercury cathode potential of betwen -1.7 and l.8 volts (S.C.E.). As the electrolysis proceeded, no gas evolution was evident. After two hours at a nominal current level of 2.0 amp., the total current consumed was 0.13 Faraday. The reaction was worked up as described previously. Although some adiponitrile was produced, large amounts of fl-hydroxypropionitrile were also formed.

EXAMPLE 4 An acidic sample (252 g.) of B-chloropropionitrile was purified by washing with 500 ml. of 10% sodium carbonate solution. Three hundred milliliters of methylene chloride was added to the mixture to facilitate separation. The lower methylene chloride layer was separated, dried over anhydrous potassium carbonate and distilled. After removal of methylene chloride, 220 g. of neutral )8- chloropropionitrile was obtained, B.P. 5055 (8 .mm.).

A catholyte was prepared by combining 60 g. of tetramethylammonium hydroxide pentahydrate, 50 g. of distilled water, and 63 g. of p-toluenesulfonic acid monohydrate. The pH was adjusted to 7, and 40 g. of purified B-chloropropionitrile (0.447 mole) was added. The anolyte was composed of 12 g. of the hydroxide, 10 g. distilled water, and 12 g. of the acid. The electrolysis was started and the cathode potential was maintained between 1.9 and 1.8 volts (S.C.E.). The reaction mixt-ure became acidic during electrolysis and there was slight gas evolution. Tetramethylammonium hydroxide pentahydrate and tetramethylammonium acetate were added to maintain the pH of the solution between 5 and 6. During 0.5 hr., 1 6 g. hydroxide and 5 g. acetate were added. Upon completion of the electrolysis, the product was worked up as described in Example 1. Distillation gave Fraction 1, B.P. 41-43, 334.2 g., which was mainly methylene chloride solvent; and intermediate Fraction 2, B.P. 43-50, 2.3 g., and a remainder (Fraction 3), 29.3 g. Vapor phase chromatographic analysis of Fractions 1-3 indicated the presence of the following: acry- 7 EXAMPLE A dimerization was carried out with the electrolyte proportions described in Example 1 and 40.0 g. (0.447 mole) of B-chloropropionitrile. Before beginning the electrolysis, 5.0 g. of 50% tetramethylammonium acetate was added to the electrolyte. The electrolysis, carried out at l.9 v., at a temperature of 39-28 and a pH of 5.5, was interrupted after 0.1 Faraday was consumed. One tenth of one mole of fi-chloropropionitrile was consumed, an indication of the consumption of one electron per molecule of fi-chloropropionitrile under conditions of low conversion.

In the table which follows, the left-hand column lists additional reactants which, when reacted under the conditions of Example 1, give the corresponding products of the right-hand column.

Reactants Dimeric Product BrCHzCHzCN NC(CH2)4CN ICHzCHzCN NC(CH2)4ON ti r ClCHzCHCN (NCOHCH2)2 CeHu CeHn l l ClCHz- HON (NC HCH2)2 (NC HCHZ) 2 The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The process of preparing an adiponitrile of the formula (CH CHRCN) which comprises electrolyzing, at a cathode potential of about -1.5 to

-2.3 volts as referred to a saturated calomel electrode and at a pH of about 5 to 7,

an aqueous catholyte containing initially about 540% by weight, based on the weight of the catholyte excluding B-halonitrile, of at least one ,B-halopropionitrile of the formula XCH CHRCN, wherein:

X is halogen of atomic number 17-53 and R is selected from the group consisting of hydrogen, alkyl of 1-8 carbons, phenyl and o-, mand p-substituted phenyl, the substituents being CN, Cl, Br, or alkyl or alkoxy of 1-8 carbons.

2. The process of claim 1 employing an aqueous electrolyte containing about 10-50% by weight, based on the weight of the catholyte excluding ,B-halonitrile, of a salt free from reactive groups and sufliciently stable to remain substantially in aqueous solution.

3. The process of claim 2 in which the salt is selected from the group consisting of the amine and quaternary ammonium salts of aryl and alkaryl sulfonic acids.

4. The process of claim 1 in which adiponitrile is produced by the electrolysis of p-chloropropionitrile.

References Cited UNITED STATES PATENTS 3,193,481 7/1965 Baizer 20473 OTHER REFERENCES Trans. Electrochem. Soc., vol. 73, pp. 524-527 and 534-537, 1938.

Trans. Electrochem. Soc., vol. 92, pp. 391-400, 1947.

Allen, Organic Electrodes Potentials, pp. 91-92.

JOHN H. MACK, Primary Examiner H. M. FLOURNOY, Assistant Examiner 

